There are a lot of similarities between writing music and writing essays. The Intro-Body-Conclusion form is called in music Exposition-Development-Recapitulation. Sometimes there's an introduction before that, but most of the time it just starts at the beginning of the first theme. The Exposition is the first section, where the main themes or tunes are introduced. Those main themes are like the thesis in the essay. A tune is a musical idea, just like a thesis is a word idea. The Development is the section where the themes are developed and changed. That's like the Body section of the essay, where you develop on the thesis you stated in the Introduction and give supporting arguments for it. In the Development section you do things to themes like play them backwards, change notes or harmonies, and play two themes together. In the Body of an essay you might do things to a thesis like support it with arguments, compare/contrast it with another idea, or cite other sources to expand on it. In a novel, an author might use such tools as foreshadowing, flashbacks, and figures of speech like metaphors and similes. In the Recapitulation part, the theme is restated, maybe with a few changes, like you might restate your thesis with another supporting argument. Then (in the music) there might be a Coda, or 'tail,' to make the ending sound more final. This would also be likened to the Conclusion.
